Sony BVS-3100 Beginner Question - Button Flashing

I have a BVS-3100 switcher with several inputs, one of them being a color bar generator. (Note that this switcher appears to be the same as the Grass Valley 110 switcher.)

When I select the color bar generator on the program bus the button flashes but the color bars appear on the program monitor just fine. If I select 'color background' on the program bus that button doesn't flash (and the color background appears on the program monitor just fine).

If I select the color bars on the preview bus the button doesn't flash and the color bars appear on the preview monitor just fine.

I have a PDF of the manual but nothing in it says what is indicated by a flashing button.

Does anyone know what is indicated by a flashing button ?

Thanks in advance.